Wet and dry etching process on &lt;110&gt; silicon and resulting structures

ABSTRACT

A method of producing smooth sidewalls on a micromachined device is described. A portion of the wafer is dry etched, forming a dry etched sidewall. The sidewall is covered with a mask. An area adjacent to the dry etched area is wet etched, forming a wet etched sidewall. The mask may optionally be removed after wet etching. The wafer substrate has a &lt;110&gt; orientation, which allows the wet etched area to have nearly vertical wet etched sidewalls.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from provisional application serial No. 60/285,152, filed Apr. 20, 2001, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention generally relates to formation of micromachined devices having smooth surfaces by utilizing a combination of both wet etching and dry etching, such as deep reactive ion etching.

BACKGROUND

[0003] Deep reactive ion etching (RIE) allows for the creation of intricately etched components in a micromachined device. Examples of such intricately etched components include flexures and actuators, such as comb drives and cantilevers. Conventional dry etching methods, such as RIE, produce sidewalls with surfaces that are too rough for use as optical surfaces. Thus, a new etching methodology is required which allows for both the creation of intricately etched components and the creation of smooth sidewalls.

SUMMARY

[0004] The invention provides micromachined devices that have portions with sidewalls smooth enough to be used in optical devices, such as mirrors, filters and windows. In one aspect, the invention provides a method of forming an anisotropically wet etched sidewall adjacent to a dry etched sidewall in a <110> substrate. The method includes the steps of directionally dry etching into an area of a <110> oriented substrate to form a dry etched sidewall, coating the dry etched sidewall with a mask material resistant to an anisotropic wet etchant, and anisotropically wet etching an area adjacent to the dry etched sidewall to form a wet etched sidewall.

[0005] The invention further provides a micromachined device formed in a <110> crystal substrate that includes a dry etched sidewall and an anisotropically wet etched sidewall adjacent to the dry etched sidewall.

[0006] These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention that is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1a is a perspective view of a micromachined device constructed in accordance with an embodiment of the invention.

[0008]FIG. 1b is a cross-sectional view taken along line Ib-Ib of FIG. 1a.

[0009] FIGS. 2-16 are views showing the fabrication of a portion of the micromachined device of FIG. 1.

[0010] FIGS. 17-18 are top views of the micromachined device of FIG. 1.

[0011] FIGS. 19-26 are views showing the fabrication of a portion of a micromachined device in accordance with another embodiment of the invention.

[0012] FIGS. 27-28 are cross-sectional views of a portion of a micromachined device constructed in accordance with an embodiment of the invention.

[0013]FIG. 29 is a top view of a micromachined device constructed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0014] Referring now to FIGS. 1-19, there is described a method for producing smooth sidewalls in complex micromachined devices. As shown in FIGS. 1a-1 b, a micromachined device 50 includes an active portion 40 on a substrate 32. The active portion 40 includes a pair of structures, namely a stationary portion 30 and a flexure 28. The flexure 28 has two parts sandwiching a pedestal 29, which includes a pair of smooth sidewalls 22 _(a) and 22 _(b), each located on a surface of an expanded portion of the pedestal 29. The flexure 28 is released from the substrate 32. The stationary portion 30 is attached to the substrate 32 with a release layer 115 (FIG. 1b). The stationary portion 30 includes another smooth sidewall 22 _(c) located on a surface of an expanded portion of a second pedestal 31. An etalon spacing 26 separates the two smooth sidewalls 22 _(b) and 22 _(c). On either side of the etalon spacing 26 between the stationary portion 30 and the flexure 28 are a pair of comb drives 24 _(a), 24 _(b). As used herein, smooth means that the sidewalls 22 _(a), 22 _(b), 22 _(c) have a texture which is not as rough as that obtained through a dry etch process and which may be sufficiently smooth to be of optical quality.

[0015] The mechanical flexure 28 may be electrostatically actuated by the comb drives 24 _(a), 24 _(b). The stationary portion 30 is bonded to the substrate 32; however, the flexure 28 is released from the substrate 32. The smooth sidewalls 22 _(a), 22 _(b), 22 _(c) of the stationary portion 30 and the flexure 28 provide a Fabry Perot Etalon which is tunable by adjusting the voltage on the comb drives 24 _(a), 24 _(b). Adjusting the voltage on the comb drives 24 _(a), 24 _(b) varies the etalon spacing 26.

[0016] The smooth sidewalls 22 _(a), 22 _(b), 22 _(c) must be formed with a sufficient smoothness that cannot adequately be obtained through dry etching, such as, for example, deep reactive ion etching (RIE), alone. Thus, the smooth sidewalls 22 _(a), 22 _(b), 22 _(c) are formed through anisotropic wet etching. FIG. 1b illustrates a cross-sectional view of the micromachined device 50. The smooth sidewalls 22 _(a), 22 _(b), 22 _(c) are all parallel to each other and the flexure 28 may be moved toward or away from the stationary portion 30 in the direction A.

[0017] Next will be described the processing steps to create the smooth sidewalls 22 _(a), 22 _(b), 22 _(c). FIGS. 2-4 show a generalized description of the technique. As shown in FIGS. 2-4, a mask 12 is positioned on a <110> silicon wafer portion 10. The mask 12 as shown approximates the shape of a letter T. A hard mask 14 covers areas to be wet etched. It should be understood, however, that the areas beneath the hard mask 14 may be any shape. A more optimal shape, a 54.7° parallelogram, can be lined up with the <111> planes of the silicon wafer portion 10, thereby allowing for the wet etching of a smooth sidewall. The remainder 16 of the wafer portion 10 is exposed. The mask 12 and the hard mask 14 are formed of different, selectively etchable materials. For example, if the mask 12 is formed of silicon dioxide, the hard mask 14 may be formed of nitride. The mask 12 instead may be formed of silicon nitride or tantalum oxide (Ta₂O₅).

[0018] Next, with reference to FIGS. 5-7, the exposed portions 16 of the wafer 10 are dry etched. A deep reactive ion etching (RIE) may be used to etch the exposed portions 16, leaving an open portion 16′. Alternatively, other suitable dry etching processes may be utilized, such as, for example, ion beam milling, laser-chemical etching, laser ablation, or laser drilling. The dry etch may extend through the wafer 10, or alternatively, down to the release layer 115. Then, as shown in FIGS. 8-10, oxidation of the masked portion and the sidewalls of the masked portion is performed, leaving oxidized areas 18. The oxidization may be performed by a thermal oxidation. The resulting oxidized areas 18 protect the sidewalls from subsequent anisotropic wet etching. As shown in FIGS. 9-10, the hard mask 14 patterns where the oxide grows. Provided

[0019] the mask 12 is formed from an oxide, the oxidization thickens the mask 12 in areas not covered by the hard mask 14.

[0020] After preparing the oxidized areas 18, a wet or dry etch is performed to remove the hard mask 14 (FIGS. 11-13). After removal of the hard mask 14, the oxide is thinned by a timed partial etch, in hydrogen fluoride for example, which exposes the silicon. At this point in the processing, the wafer 10 is covered by an oxide in areas not covered by the hard mask 14 and not originally exposed. Then, a wet anisotropic, e.g. Tetramethyl Ammonium Hydroxide (TMAH) or Potassium Hydroxide (KOH), etch is performed to create the smooth sidewalls 22 _(a), 22 _(b), 22 _(c) (FIGS. 14-16). In instances where the mask 12 is formed of silicon dioxide, TMAH is preferred over KOH because TMAH does not attack silicon dioxide. Potassium hydroxide (KOH) attacks silicon dioxide, but not silicon nitride. The smooth sidewalls are generically denoted as sidewall 22 in FIG. 15. The sidewalls 22 may be vertical or nearly vertical, meaning within plus or minus ten to thirty degrees relative to the substrate surface. Alternatively, the sidewalls 22 may be wet etched nonvertically by using wafers that are cut at an angle with respect to the <110> plane. It is important for the mask 12 to be lined up on the <111>

[0021] plane to ensure that smooth sidewalls 22 are formed. If the mask 12 is not properly lined up on the <111> plane, the sidewalls may be formed insufficiently smooth.

[0022] The wet etch leaves a remaining silicon wedge 23 adjacent the sidewall 22 (FIG. 16). The wedge 23 is defined by nonvertical <111> planes in the silicon. In most cases, the device should be designed so that the wedges 23 fall away when the oxide 18 on the sidewalls is removed since the wedges 23 are supported by the oxide 18 on the sidewalls. Specifically, without the presence of the pedestals 29, 31, the wedges 23 would be affixed to the sidewalls 22 _(a), 22 _(b), 22 _(c).

[0023] FIGS. 17-18 illustrate the micromachined device 50 of FIG. 1 indicating portions between the two comb drives 24 _(a), 24 _(b) which are wet etched to create the smooth sidewalls 22 _(a), 22 _(b), 22 _(c). The areas generally denoted by the remainder 16 include dry-etch areas and encompass the two comb drives 24 _(a), 24 _(b). Specifically, the areas 16 are dry etched to form the comb drives 24 _(a), 24 _(b). The areas generally covered by the hard mask 14 are wet-etched areas which define the smooth sidewalls 22 _(a), 22 _(b), 22 _(c). The mask 12 covers the flexure 28. After wet etching the wet-etched areas 14, the wedges 23 remain supported by the oxide 18. Removing the oxide 18 allows the wedges 23 to drop away. If the pedestals 29, 31 did not include expanded areas beyond the flexure 28 and the stationary portion 30, respectively, the wedges 23 would remain affixed to the sides of the flexure 28 and the stationary portion 30 and would have to be etched away, possibly affecting the smoothness of the sidewalls 22 _(a), 22 _(b), 22 _(c).

[0024] An alternative processing embodiment of the invention is illustrated in FIGS. 19-26. As particularly shown in FIG. 19, a <110> silicon wafer 110 has a body 116 overlying a release layer 115 and a substrate 32. The body 116 may be formed of silicon, and the release layer 115 may be formed of silicon dioxide. A hard mask 114 is deposited in a pattern on the body 116. A mask 112, namely an oxide layer, is then deposited over the hard mask 114 and the body 116. The portion of the substrate above a dry-etched area 119 remains free of either the mask 112 or the hard mask 114. A wet-etched area 117 is covered by the mask 112 alone.

[0025] The dry-etched area 119 is then etched by any suitable dry etching method, such as, for example, reactive ion etching, ion milling, or any other similar process (FIG. 20). The etching of the dry-etched area 119 extends down to the release layer 115. The wafer 110 is then conformally coated with a chemical vapor deposition of a silicon nitride 121 (FIG. 21). The deposition of silicon nitride 121 occurs over the mask 112, along the sidewalls of the dry etch area 119, and along the release layer 115. It should be appreciated that any conformal hard mask that is resistant to anisotropic wet etchant may be used in lieu of silicon nitride 121.

[0026] Then the wafer 110 is planarized, polished, or etched to remove the layer of deposited nitride 121 from above the mask 112 only (FIG. 22). Then, as shown in FIG. 23, the mask 112 is removed through the use of a dilute solution of hydrogen fluoride (HF) or other suitable solvent or etchant. The removal of the mask 112 leaves the wet-etched area 117 exposed.

[0027] The wet-etched area 117 is then wet etched (FIG. 24). The hard mask 114 may be made of a nitride compound, and thus the wet-etched area 117 may be wet etched with potassium hydroxide (KOH), which will create a smooth sidewall 122 along a side in the area of the substrate still covered by the hard mask 114. The sidewall 122 may be vertical or nearly vertical, e.g., ±ten to thirty degrees relative to the substrate surface. As an optional processing step, the hard mask 114 and the nitride 121 may be removed with an etchant that does no damage to the substrate 132 (FIG. 26), leaving the smooth sidewall 122 opposite a rougher sidewall 124 formed via RIE. Optionally, the release layer 115 can be etched to release a structure formed from the body 116.

[0028] Both of the above-described etching processes may be used to create a variety of optical devices, including etalons, beamsteering devices, movable mirrors, and filters. Further, smooth surfaces may be combined with out-of-plane flexures to form out-of-plane reflectors, as described below.

[0029]FIG. 26 shows a top view of a micromachined device 150 formed by the process described in reference with FIGS. 19-25. The comb drives 24 _(a), 24 _(b) are formed by dry etching the areas E. The areas D are exposed during a wet etching step. It is important, though not essential, that the wet etched areas D have a width 118 that is wider than the width S₁ of the vertical sidewall 122 so as to provide wedges 23 that are not attached to the flexure 28. The shape of the wedges 23 depend greatly on the shape of the wet etched area 118. The remaining wedges 23 are superimposed over the wet etched areas 118 to illustrate a possible positioning of the wedges 23. Through such positioning, the wedges 23 are not attached to the flexure 28. In fact, after wet etching, the wedges 23 will only be supporting by the freestanding sidewall protection layer, e.g. the freestanding silicon nitride 121 shown in FIG. 24.

[0030] FIGS. 27-28 schematically illustrate an alternative micromachined device that may be fabricated to include a flexure 128 that may be rotated relative to the stationary portion 30 in either the direction B (FIG. 27) or the direction C (FIG. 28). Such rotation may allow the flexure 128 to be used in a beamsteering device.

[0031] A micromachined device 250, which is constructed in accordance with another embodiment of the invention, is shown in a top view in FIG. 29. The micromachined device 250 includes a flexure 228, having a pedestal 29, and a stationary portion 230 attached to a substrate 32. The pedestal 29 includes a pair of expanded portions which have smooth sidewalls 22 _(a) and 22 _(b). The pedestal 29 has a width W₂, which is greater in width than a width W₁ of the flexure 228 by the distance of the expanded portions. It should be understood that the pedestal 29 may have only one expanded portion instead of two. Between the flexure 228 and the stationary portion 230 is a comb drive 224 made up of comb elements extending off of both the flexure 228 and the stationary portions 230 and intermeshed one with another. The micromachined device 250 may be utilized as a tilt-tunable etalon, a beam scanner/deflector, or a movable mirror, for example.

[0032] While the invention has been described in detail in connection with exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

What is claimed as new and desired to be protected by Letters Patent of the United States is:
 1. A method of forming an anisotropically wet etched sidewall adjacent to a dry etched sidewall in a <110> substrate, said method comprising: directionally dry etching into an area of a <110> oriented substrate to form a dry etched sidewall; coating the dry etched sidewall with a mask material resistant to an anisotropic wet etchant; and anisotropically wet etching an area adjacent to the dry etched sidewall to form a wet etched sidewall.
 2. The method of claim 1, further comprising removing the mask material after said anisotropically wet etching.
 3. The method of claim 1, wherein said anisotropically wet etching forms a <111> sidewall at an angle of between ten and thirty degrees from a surface of the <110> substrate.
 4. The method of claim 1, wherein the substrate comprises silicon.
 5. The method of claim 1, wherein the mask material comprises one or more of the group consisting of silicon dioxide, silicon nitride, and tantalum oxide.
 6. The method of claim 1, wherein the dry etched sidewall and the wet etched sidewall are disposed on a flexure.
 7. The method of claim 6, wherein the area being anisotropically wet etched extends a distance greater than the width of the wet etched sidewall.
 8. The method of claim 7, wherein the wet etched sidewall is disposed on a pedestal.
 9. The method of claim 7, wherein the pedestal is of greater width than the flexure.
 10. The method of claim 1, further comprising removing remaining wedges after said anisotropically wet etching.
 11. A micromachined device formed in a <110> crystal substrate, comprising: a dry etched sidewall; and an anisotropically wet etched sidewall adjacent to said dry etched sidewall.
 12. The micromachined device of claim 11, wherein said wet etched sidewall is formed from a larger wet etched area.
 13. The micromachined device of claim 11, wherein said wet etched sidewall is located on a pedestal.
 14. The micromachined device of claim 13, wherein said pedestal is located on a structure.
 15. The micromachined device of claim 14, wherein said pedestal comprises a second wet etched sidewall opposite said wet etched sidewall.
 16. The micromachined device of claim 14, wherein said wet etched sidewall is located on an expanded portion of said pedestal, said pedestal being wider than said structure.
 17. The micromachined device of claim 16, wherein said structure is a stationary portion.
 18. The micromachined device of claim 16, wherein said structure is a flexure.
 19. The micromachined device of claim 18, wherein said pedestal is located at an end of said flexure.
 20. The micromachined device of claim 18, wherein said pedestal is located in the middle of said flexure.
 21. The micromachined device of claim 18, further comprising an electrostatic actuator for moving said flexure.
 22. The micromachined device of claim 21, wherein said electrostatic actuator comprises a comb drive.
 23. The micromachined device of claim 21, wherein said electrostatic actuator comprises a cantilever.
 24. The micromachined device of claim 18, further comprising a stationary portion spaced apart from said flexure.
 25. The micromachined device of claim 24, wherein a space between said flexure and said stationary portion may be altered.
 26. The micromachined device of claim 24, wherein said flexure may be rotated relative to said stationary portion.
 27. The micromachined device of claim 24, wherein said stationary portion comprises a second pedestal, said second pedestal being wider than said stationary portion.
 28. The micromachined device of claim 27, wherein said second pedestal comprises a second wet etched sidewall spaced apart from said wet etched sidewall.
 29. The micromachined device of claim 11, wherein said wet etched sidewall is defined by a nearly vertical <111> plane.
 30. The micromachined device of claim 11, wherein said wet etched sidewall comprises an optical quality sidewall.
 31. The micromachined device of claim 11, wherein said substrate is a single crystal silicon substrate. 